[raiseCatError]

Closes the last unbuilt item of #463's implementation scope (the issue stays open for physical QA).

• Renumber footnotes (snippet palette): numbered footnotes become 1, 2, 3… in order of first reference; definitions follow; one undo step; only labels change.
• Named footnotes, escaped references, code spans and fenced code untouched; unreferenced definitions keep their number unless needed, then take the next free one.

Tests: footnotes.test.ts (6) + helper/snippet/editor suites. Not run: full suite, physical QA.
Refs #463.